Barber pole structure for magnetorestrictive sensors

ABSTRACT

A method of making a magnetorestrictive sensor involves the deposition of a magnetorestrictive strip over a substrate, the deposition of an insulating layer over the magnetorestrictive strip, the etching of barber pole windows through the insulating layer, the deposition of a conductive material over the insulating layer and into the barber windows, and the etching away of the conductive material between the barber pole windows so as to form barber poles. In this manner, the formation of the barber poles is controlled by the windows formed in the insulating layer.

RELATED APPLICATION

[0001] U.S. patent application Ser. No. (B10-17305) discloses subjectmatter which is similar to the subject matter disclosed herein.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates in general to magnetic fieldsensors and, more particularly, to magnetorestrictive sensors.

BACKGROUND OF THE INVENTION

[0003] Magnetorestrictive sensors are typically small and can generallymeasure magnetic fields on the order of 0.001 gauss to 100 gauss. Also,magnetorestrictive sensors are able to measure D.C. fields as well asfields having frequencies up to and exceeding a megahertz. Accordingly,magnetorestrictive sensors are used in a wide variety of applicationssuch as current sensing, proximity sensing, etc.

[0004] The magnetorestrictive material used in making magnetorestrictivesensors is a material whose resistance changes in the presence of amagnetic field. Permalloy, which is a nickle/iron alloy, is such amaterial and is often provided as a film for use in magnetorestrictivesensors. The resistance of the film varies according to the square ofthe cosine of the angle between the magnetization direction and thecurrent direction.

[0005] The response of a magnetorestrictive material is measured asΔR/R_(N), where ΔR is the change in resistance of the magnetorestrictivematerial and R_(N) is the nominal resistance of the magnetorestrictivematerial. The change in the resistance ΔR of Permalloy between the pointwhere the magnetization direction is parallel to the current directionand the point where the magnetization direction is perpendicular to thecurrent direction is on the order of 2% of the nominal resistance of thematerial.

[0006] Moreover, the plot of ΔR/R_(N) versus the angle between themagnetization direction and the current direction is bell shaped. Inorder to operate the magnetorestrictive material on the linear part ofthis curve, a bias field is frequently applied to the magnetorestrictivesensor. For example, either a solenoid wrapped around themagnetorestrictive sensor package or a plurality of thin-film permanentmagnets at the end of the magnetorestrictive sensor are usually used tobias the magnetorestrictive material at this linear portion.

[0007] Alternatively, instead of applying a biasing field to themagnetorestrictive sensor, it is known to provide the magnetorestrictivesensor with conductive barber poles. Unlike the bias field which rotatesthe magnetization direction with respect to the current direction,barber poles instead rotate the current direction with respect to themagnetization direction.

[0008] Magnetorestrictive sensors are frequently used in Wheatstonebridges. Thus, each of the four legs of a Wheatstone bridge contains amagnetorestrictive sensor. A top view of an exemplary known Wheatstonebridge 10 is shown in FIG. 1. The Wheatstone bridge 10 includes fourmagnetorestrictive sensors 12, 14, 16, and 18. The magnetorestrictivesensor 12 is formed from a Permalloy film 20 and has a set of barberpoles 22 for biasing. The magnetorestrictive sensor 14 is formed from aPermalloy film 24 and has a set of barber poles 26 for biasing. Themagnetorestrictive sensor 16 is formed from a Permalloy film 28 and hasa set of barber poles 30 for biasing. Finally, the magnetorestrictivesensor 18 is formed from a Permalloy film 32 and has a set of barberpoles 34 for biasing.

[0009] The nominal resistances of the four legs are ideally identical sothat the Wheatstone bridge 10 is balanced and has no output in theabsence of a magnetic field. The use of a bias field to operate themagnetorestrictive material forming the magnetorestrictive sensors 12,14, 16, and 18 in the Wheatstone bridge 10 at the linear portion of themagnetization/current curve does not upset this balance because theresistances of the four sensors change by the same amount in response tothe bias field.

[0010] The use of barber poles, however, can upset this balance, such aswhere the barber poles are not uniformly formed over themagnetorestrictive material of the four sensors. As described above, animbalance causes the bridge to have an output even when no magneticfield is present. This output is usually referred to as offset.Accordingly, it is known to reduce this offset through the use of lasertrimming. However, laser trimming adds cost to devices such asWheatstone bridges which use magnetorestrictive sensors.

[0011] The present invention is directed, at least in one embodiment, toan arrangement of barber poles which allows tighter control of offsetand, therefore, reduces the need for laser trimming.

SUMMARY OF THE INVENTION

[0012] In accordance with one aspect of the present invention, a methodof making a magnetorestrictive sensor comprises the following: forming adielectric over a magnetorestrictive material; forming barber polewindows through the dielectric; and, forming barber poles through thebarber pole windows.

[0013] In accordance with another aspect of the present invention, amethod of making a magnetorestrictive sensor comprises the following:depositing a magnetorestrictive strip over a substrate; depositing aninsulating layer over the magnetorestrictive strip; etching barber polewindows through the insulating layer; depositing a conductive materialover the insulating layer and into the barber windows; and, etching awaythe conductive material between the barber pole windows so as to formbarber poles.

[0014] In accordance with yet another aspect of the present invention, amagnetorestrictive sensor comprises a substrate, a magnetorestrictivestrip over the substrate, barber poles of conductive material over themagnetorestrictive strip, and a dielectric between adjacent ones of thebarber poles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other features and advantages will become more apparentfrom a detailed consideration of the invention when taken in conjunctionwith the drawings in which:

[0016]FIG. 1 shows a known Wheatstone bridge having magnetorestrictivesensors formed of Permalloy (NiFe) films and barber poles;

[0017]FIG. 2 shows a Permalloy layer and a Tantalum nitride (TaN) layerresulting from preliminary steps in the formation of amagnetorestrictive sensor according to one embodiment of the presentinvention;

[0018]FIG. 3 shows the magnetorestrictive sensor following a Tantalumnitride etch according to one embodiment of the present invention;

[0019]FIG. 4 shows the magnetorestrictive sensor following a Permalloyetch and a sputter nitride deposition according to one embodiment of thepresent invention;

[0020]FIG. 5 shows the magnetorestrictive sensor following a nitrideetch according to one embodiment of the present invention;

[0021]FIG. 6 shows the magnetorestrictive sensor following deposition ofa conductive layer according to one embodiment of the present invention;

[0022]FIG. 7 shows the barber poles of the magnetorestrictive sensorfollowing selective etching of the conductive layer according to oneembodiment of the present invention; and,

[0023]FIG. 8 shows the barber poles of a magnetorestrictive sensorresulting from conventional processing.

DETAILED DESCRIPTION

[0024] A magnetorestrictive sensor 40 as illustrated in FIG. 2 isproduced by depositing a thermal oxide layer 42 over a silicon substrate44. (The view in FIG. 2 is a side view of one magnetorestrictivesensor.) The thermal oxide layer 42 is a dielectric layer used toelectrically isolate the silicon substrate 44 from the rest of themagnetorestrictive sensor 40 and may have a thickness of, for example,4000

. A nitride layer 46 is sputtered over the thermal oxide layer 42. Thenitride layer 46 may have a thickness of, for example, 300

. Over the nitride layer 46 is deposited a Permalloy layer 48 to athickness, for example, of 175

, and a tantalum nitride layer 50 is deposited over the Permalloy layer48 to a thickness, for example, of 600

.

[0025] The nitride layer 46 provides an atomically smooth surface forthe Permalloy layer 48, and creates a barrier between the thermal oxidelayer 42 and the Permalloy layer 48. The tantalum nitride layer 50provides a good hard etch mask and a good barrier that allows only asmall amount of current to flow through it with most of the currentflowing through the upper conductive layers of the magnetorestrictivesensor 40.

[0026] As illustrated in FIG. 3, the tantalum nitride layer 50 isselectively etched so that the portions of tantalum nitride layer 50over the ends of the Permalloy layer 48 are removed. As illustrated inFIG. 4, the exposed portions of the Permalloy layer 48 are etched awayso that the Permalloy layer 48 is substantially commensurate with theTantalum nitride layer 50 and so that the exposed portions of thenitride layer 46 are partially etched. Then a dielectric layer 52, suchas silicon dioxide or silicon nitride, is sputtered over the tantalumnitride layer 50 and the exposed and partially etched portions of thenitride layer 46. The dielectric layer 52 may have a thickness, forexample, of 800

.

[0027] As shown in FIG. 5, the dielectric layer 52 is selectively etchedto form windows 53 down to the tantalum nitride layer 50. The pattern ofthe windows 53, which are formed by the remaining dielectric layer 52,defines the barber poles that are formed by subsequent processing. Asshown in FIG. 6, this subsequent processing includes the deposition of aconducting layer 54, such as aluminum copper (AlCu), over the dielectriclayer 52 and through the windows 53 in the dielectric layer 52 down tothe tantalum nitride layer 50. The conducting layer 54 may have athickness, for example, of 5000

.

[0028] Finally, as shown in FIG. 7, the conducting layer 54 between thewindows is selectively etched to form barber poles 56. Each of thebarber poles 56 is insulated from an adjacent barber pole bycorresponding portions of the dielectric layer 52. The barber poles 56may be compared to known barber poles 58 which are shown in FIG. 8.

[0029] The process of defining the barber poles 56 by use of the windows53 formed in the dielectric layer 52 allows the shape and size of thebarber poles 56 to be controlled better than where the size and shape ofthe barber poles are controlled by the etching of the barber pole metalitself, as is the case with the magnetorestrictive sensor shown in FIG.8. Accordingly, the present invention leads to tighter control of bridgeoffset and less reliance on laser trimming. Moreover, the presentinvention results in better sensor bridge resistance control because thedielectric layer 52 permits better definition of the contact areabetween the barber pole metal and the Permalloy layer and because thedielectric layer 52 permits smoother barber pole edges.

[0030] Certain modifications of the present invention will occur tothose practicing in the art of the present invention. For example, themagnetorestrictive sensor 40 is shown with each of the layers 42, 44,46, 48, 50, 52, and 54. However, one or more of these layers may beomitted or may be replaced by other layers, and/or additional layers maybe provided.

[0031] Accordingly, the description of the present invention is to beconstrued as illustrative only and is for the purpose of teaching thoseskilled in the art the best mode of carrying out the invention. Thedetails may be varied substantially without departing from the spirit ofthe invention, and the exclusive use of all modifications which arewithin the scope of the appended claims is reserved.

What is claimed is:
 1. A method of making a magnetorestrictive sensorcomprising: forming a dielectric over a magnetorestrictive material;forming barber pole windows through the dielectric; and, forming barberpoles through the barber pole windows.
 2. The method of claim 1 whereinthe magnetorestrictive material comprises Permalloy.
 3. The method ofclaim 1 wherein the formation of the dielectric comprises depositing thedielectric over the magnetorestrictive material.
 4. The method of claim3 wherein the formation of the barber pole windows through thedielectric comprises selectively etching the dielectric to form thebarber pole windows.
 5. The method of claim 4 wherein the formation ofthe barber poles comprises depositing conductors through the barber polewindows.
 6. The method of claim 5 wherein the deposition of theconductors through the barber pole windows comprises depositing aconductive material over the dielectric and the barber pole windows andetching the conductive material to separate the barber poles.
 7. Themethod of claim 1 wherein the formation of the barber pole windowsthrough the dielectric comprises selectively etching the dielectric toform the barber pole windows.
 8. The method of claim 7 wherein theformation of the barber poles comprises depositing conductors throughthe barber pole windows.
 9. The method of claim 8 wherein the depositionof the conductors through the barber pole windows comprises depositing aconductive material over the dielectric and into the barber pole windowsand etching the conductive material to separate the barber poles. 10.The method of claim 1 wherein the formation of the barber polescomprises depositing conductors through the barber pole windows.
 11. Themethod of claim 10 wherein the deposition of the conductors through thebarber pole windows comprises depositing a conductive material over thedielectric and into the barber pole windows and etching the conductivematerial to separate the barber poles.
 12. The method of claim 1 whereinthe formation of the barber poles comprises depositing a conductivematerial over the dielectric and into the barber pole windows andetching the conductive material to separate the barber poles.
 13. Amethod of making a magnetorestrictive sensor comprising: depositing amagnetorestrictive strip over a substrate; depositing an insulatinglayer over the magnetorestrictive strip; etching barber pole windowsthrough the insulating layer; depositing a conductive material over theinsulating layer and into the barber windows; and, etching away theconductive material between the barber pole windows so as to form barberpoles.
 14. The method of claim 13 wherein the deposition of themagnetorestrictive strip over the substrate includes depositing atantalum nitride strip over the magnetorestrictive strip, and whereinthe deposition of the insulating layer comprises depositing theinsulating layer over the tantalum nitride layer.
 15. The method ofclaim 14 wherein the magnetorestrictive strip has ends, wherein thetantalum strip has ends, wherein the method further comprises etchingaway the ends of the tantalum nitride strip and the magnetorestrictivestrip, and wherein the deposition of the insulating layer comprisesdepositing the insulating layer over the tantalum nitride strip and intoareas left by the etching away of the ends of the tantalum nitridestrip.
 16. The method of claim 15 further comprising depositing athermal oxide layer over the silicon substrate, wherein the depositionof the magnetorestrictive strip comprises depositing themagnetorestrictive strip over the thermal oxide layer.
 17. The method ofclaim 15 further comprising depositing a nitride layer over the siliconsubstrate, wherein the deposition of the magnetorestrictive stripcomprises depositing the magnetorestrictive strip over the nitridelayer.
 18. The method of claim 15 further comprising depositing athermal oxide layer over the silicon substrate and depositing a nitridelayer over the thermal oxide layer, wherein the deposition of themagnetorestrictive strip comprises depositing the magnetorestrictivestrip over the nitride layer.
 19. The method of claim 13 wherein themagnetorestrictive strip has ends, wherein the method further comprisesetching away the ends of the magnetorestrictive strip, and wherein thedeposition of the insulating layer comprises depositing the insulatinglayer over the magnetorestrictive strip and into areas left by theetching away of the ends of the magnetorestrictive strip.
 20. The methodof claim 19 further comprising depositing a thermal oxide layer over thesilicon substrate, wherein the deposition of the magnetorestrictivestrip comprises depositing the magnetorestrictive strip over the thermaloxide layer.
 21. The method of claim 19 further comprising depositing anitride layer over the silicon substrate, wherein the deposition of themagnetorestrictive strip comprises depositing the magnetorestrictivestrip over the nitride layer.
 22. The method of claim 19 furthercomprising depositing a thermal oxide layer over the silicon substrateand depositing a nitride layer over the thermal oxide layer, wherein thedeposition of the magnetorestrictive strip comprises depositing themagnetorestrictive strip over the nitride layer.
 23. The method of claim13 further comprising depositing a thermal oxide layer over the siliconsubstrate, wherein the deposition of the magnetorestrictive stripcomprises depositing the magnetorestrictive strip over the thermal oxidelayer.
 24. The method of claim 13 further comprising depositing anitride layer over the silicon substrate, wherein the deposition of themagnetorestrictive strip comprises depositing the magnetorestrictivestrip over the nitride layer.
 25. The method of claim 13 furthercomprising depositing a thermal oxide layer over the silicon substrateand depositing a nitride layer over the thermal oxide layer, wherein thedeposition of the magnetorestrictive strip comprises depositing themagnetorestrictive strip over the nitride layer.
 26. Amagnetorestrictive sensor comprising: a substrate; a magnetorestrictivestrip over the substrate; barber poles of conductive material over themagnetorestrictive strip; and, a dielectric between adjacent ones of thebarber poles.
 27. The magnetorestrictive sensor of claim 26 furthercomprising a tantalum nitride strip over the magnetorestrictive strip,wherein the barber poles are over the tantalum nitride layer.
 28. Themagnetorestrictive sensor of claim 27 wherein the dielectric layer isalso over sides of the tantalum nitride strip and the magnetorestrictivestrip.
 29. The magnetorestrictive sensor of claim 28 further comprisinga thermal oxide layer between the silicon substrate and themagnetorestrictive strip.
 30. The magnetorestrictive sensor of claim 28further comprising a nitride layer between the silicon substrate and themagnetorestrictive strip.
 31. The magnetorestrictive sensor of claim 28further comprising a thermal oxide layer over the silicon substrate anda nitride layer between the thermal oxide layer and themagnetorestrictive strip.
 32. The magnetorestrictive sensor of claim 26wherein the dielectric layer is also over sides of themagnetorestrictive strip.
 33. The magnetorestrictive sensor of claim 32further comprising a thermal oxide layer between the silicon substrateand the magnetorestrictive strip.
 34. The magnetorestrictive sensor ofclaim 32 further comprising a nitride layer between the siliconsubstrate and the magnetorestrictive strip.
 35. The magnetorestrictivesensor of claim 32 further comprising a thermal oxide layer over thesilicon substrate and a nitride layer between the thermal oxide layerand the magnetorestrictive strip.
 36. The magnetorestrictive sensor ofclaim 26 further comprising a thermal oxide layer between the siliconsubstrate and the magnetorestrictive strip.
 37. The magnetorestrictivesensor of claim 26 further comprising a nitride layer between thesilicon substrate and the magnetorestrictive strip.
 38. Themagnetorestrictive sensor of claim 26 further comprising a thermal oxidelayer over the silicon substrate and a nitride layer between the thermaloxide layer and the magnetorestrictive strip.